The present invention relates broadly to a method and apparatus for mounting a lens block to an optical lens blank so that the lens blank can be processed in apparatus for generating and polishing the optical lens surfaces. In particular, the present invention relates to a method and apparatus for mounting a block to a lens blank with the block centered with respect to the optical axis of the finished lens and with recesses in the lens block aligned along a line drawn through the mechanical center of the lens blank.
In the manufacture of ophthalmic lenses, a lens blank is formed into a finished lens by grinding and polishing the lens surface to a desired curvature in accordance with the prescriptive needs of the patient. Typically, lens blanks are provided having one surface ground and finished to a spherical curvature. This surface is generally on the convex side of the lens blank. The lens blank is finished by grinding and polishing the concave side of the lens to the appropriate curvature. As a rule, the concave surface will be generated as a toric surface having different curvatures at right angles to each other. Occasionally, the concave surface will be generated as spherical. To ensure that the lens blank is oriented properly for the subsequent generation of the toric surface, it is desirable to center the lens block with respect to the ultimate optical center of the finished lens. The lens blank is mounted in the surface generation machinery by the lens block, and in the conventional prior art devices, the concave surface of the lens blank is swept by a rotating grinding wheel. Once the toric surface is generated, the lens blank is then ground and polished with standard lapping equipment utilizing abrasion tools that correspond with the toric surface generated by the grinding wheel.
A number of blocking methods and apparatus have been proposed in the prior art. In perhaps the most popular method, a metal block is secured to the finished or convex side of the lens blank with a suitable adhesive material. The block has holes aligned along a line through the optical center of the finished lens. The adhesive material may be pitch or a low melting point alloy. In the methods that utilize pitch or an equivalent adhesive the metal blocks and finished lens must be thoroughly cleaned upon completion of each surfacing operation. The cleaning of the lens block is a time-consuming and therefore costly step in the manufacture of the ophthalmic lenses. The low melting point alloy is somewhat cleaner in that the melting point is such that the alloy will melt in relatively low temperature water and be completely removed from the lens block and lens blank with minimal effort.
The prior art methods and apparatus that utilize a permanent metal lens block having mounting recesses referenced to the optical center of the finished lens have been found to be unacceptable when utilized for mounting relatively large lens blanks in the lapping and polishing apparatus. The problem has become more critical with the increasing popularity of large spectacle frames consequently requiring large ophthalmic lenses. In the lapping and polishing apparatus, the toric surface is placed on a lapping or polishing tool. The lens blank is pressed against the polishing tool and moved over the tool while a slurry of abrasive or polishing compound is introduced onto the tool. When large lens blanks are utilized and the conventional metal lens blocks are affixed to the lens blanks with respect to the optical center, pressure is applied unevenly across the large area lens blank. This results from the fact that the mechanical or geometric center of the large lens blank is typically spaced from the ultimate optical center. The lapping and polishing tool is provided with projecting members that are received with the recesses formed on the metal block along a line through the optical center. This uneven application of pressure with respect to the mechanical center of the lens blank results in uneven grinding of the toric surface. Since the toric surface is not uniformly polished unwanted prism is introduced into the lens. This unwanted prism adversely affects the optical properties of the ophthalmic lens. It is therefore desirable to apply pressure evenly to the lens blank as it is being lapped and polished.
The U.S. patent to Rudd et al 3,226,887 discloses a method and apparatus for processing of ophthalmic lenses in which a lens block is provided that allows the toric surface to be generated with respect to the optical center of the lens while at the same time permits the grinding and polishing of the toric surface about the ultimate mechanical center of finished lens. The lens block of the Rudd patent includes two separate fixtures referred to as a mechanical center ring and an optical center ring. The rings are presumably formed of a suitable metal and are adhered to the lens blank utilizing a suitable bonding material such as a low melting alloy. The mechanical ring has sockets which may be aligned with respect to a horizontal line drawn across the blank through the ultimate mechanical center of the finished lens. The mechanical center of the finished lens may differ from the mechanical center of the lens blank. Thus, to insure even pressure distribution over the lens blank during grinding and polishing, the blank edges should be ground prior to the polishing step to remove any uneven overhang. This procedure necessitates the additional time-consuming step of edge grinding prior to lapping and polishing of the lens. Also, the removal of the uneven overhang of the blank to insure balanced pressure applied to the lens blank during the lapping and polishing step could result in the polished lens blank being too small to fit in the large lens frames that are becomingly increasingly popular. The lapping and grinding tool has pins which are received within the socket such that pressure is applied to the lens blank during the lapping and polishing operation.
While dealing with the problem of uneven grinding during the lapping and of a finished lens, the Rudd method utilizes a lens block having permanent metal rings that must be subjected to the same cleaning steps after use as the prior art metal blocks. Additionally, the steps of aligning the rings with the ultimate optical and mechanical centers of the finished lens are somewhat complex and time-comsuming. As previously mentioned the Rudd method and apparatus is not particularly applicable in processing large lens blanks to fit large spectacle frames.
The present invention eliminates the disadvantages of the prior art blocking methods and apparatus in that it is a method and apparatus for applying a lens block formed entirely of a low melting point material which is applied to the lens blank and hardens to form a block centered with respect to the optical center of the finished lens and which is provided with recesses oriented along a line drawn through the mechanical center of the lens blank. The lens block formed by the method and apparatus of the present invention is thus relatively inexpensive with respect to the prior art blocks and is characterized by the ease of application and alignment of the lens block and depressions therein with respect to the optical center and mechanical center, respectively. The lens block is removed by simply heating the block such that the block material returns to a molten or liquid state.